Will Facebook replace email? I don't know. It has as good a shot as any company has ever had at dominating how people communicate, but there are a number of challenges still ahead. Since the Internet began there has always been one question that unimaginative investors and naysayers use to tell innovative companies to not do something: "Why wouldn't ______ just do it?" Or, depending on the decade, "Where is Microsoft, Google or Facebook?"
It's funny, having started a company that was founded and built during Google's height of dominance, MySpace's 15 minutes of fame and Facebook's rise, I have actually heard "Why wouldn't Google just do it?", "Why wouldn't MySpace just do it?" AND "Why wouldn't Facebook just do it?" at various points. You better believe Twitter, Foursquare and every other innovative service on the Web has dealt with the same questions. But a funny thing happened on the way to Facebook replacing Twitter: People continued using Twitter, and in increasing numbers.
The difficulty in being all things to all people is one part perception and one part product. People tend to form perceptions of products and services that dominate their behavior. The internet has infinite choices with regard to almost any service. In order to not be paralyzed by choice, people simply allow particular services to define an action they will perform. Hence Twitter is for sharing bits of content, personal or professional, Foursquare is for location-based events, etc. It's just easier to build behavior patterns. It's why the best service from a usability perspective or feature perspective is not always the service that wins. This is one factor that always confounds people outside looking in.
The other difficulty in being all things to all people is that product development gets very, very complicated. It is hard enough to design a product that will achieve mass adoption; now design a product that will achieve mass adoption but must incorporate additional functionality to service the bigger product's main function. It's just easier to evolve and iterate on a product for consumers when there is a focus on excellence for one function.
Of course, examples like Google Maps prove that expanding works when the new service truly leverages the core competence (search) and its development and innovation can be very compartmentalized. When it comes to Facebook replacing email, there is the potential for similar synergies and challenges. I am looking forward to testing the innovation.
What do you think? Drop your thoughts in the comments, and let's connect on Twitter: www.twitter.com/joemarchese